Question

Good evening.

For a project, I have to create a system. In this system, users can subscribe for a masterclass. The administrator should be able to delete users and masterclasses, but the latter only if no users has subscribed to it.

My database:

CREATE TABLE IF NOT EXISTS Speler_masterclass  (
        Sid                INT(11)                NOT NULL         AUTO_INCREMENT,
        naam                VARCHAR(50)        NOT NULL,
adres                VARCHAR(100) NOT NULL,
postcode        VARCHAR(100) NOT NULL,
woonplaats        VARCHAR(100) NOT NULL,
telefoonnr        INT(20)                NOT NULL,
        email                VARCHAR(100)        NOT NULL,
        ratingscore        INT(11)                NOT NULL DEFAULT '0',


PRIMARY KEY(Sid)
)ENGINE=InnoDb;




CREATE TABLE IF NOT EXISTS db11071230.Masterclass (
                   Mid                        INT(11)                NOT NULL        AUTO_INCREMENT,
                   naam                        VARCHAR(50)        NOT NULL,
            tijd                        VARCHAR(20)        NOT NULL,
            datum                        VARCHAR(10)        NOT NULL,
            plaats                        VARCHAR(100)        NOT NULL,
            minimale_rating        INT(11)        NOT NULL,

PRIMARY KEY (Mid)
) ENGINE=InnoDb;

CREATE TABLE IF NOT EXISTS  db11071230.Geeft_masterclass (
            Rankingspunten_cadeau        INT(11)        NOT NULL,
            Sid                                 INT(11)        NOT NULL,
            Mid                                INT(11)           NOT NULL,

FOREIGN KEY (Sid)               REFERENCES Speler_masterclass (Sid),
FOREIGN KEY (Mid)             REFERENCES Masterclass (Mid),
PRIMARY KEY (Sid, Mid)
) ENGINE=InnoDb;

CREATE TABLE IF NOT EXISTS  db11071230.Inschrijving_masterclass (
            betaling                                    INT(1)                    NOT NULL,
            Sid                                              INT(11)        NOT NULL ,
            Mid                                            INT(11)        NOT NULL ,

FOREIGN KEY (Sid)               REFERENCES Speler_masterclass (Sid),
FOREIGN KEY (Mid)             REFERENCES Masterclass (Mid)
ON DELETE RESTRICT
ON UPDATE CASCADE,
PRIMARY KEY (Sid, Mid)


) ENGINE=InnoDb;

IN this, I want to insert the following:

INSERT INTO Geeft_masterclass
        VALUES (5, 1, 1)
;
INSERT INTO Geeft_masterclass
        VALUES (6, 2, 2)
;
INSERT INTO Geeft_masterclass
        VALUES (7, 2, 2)
;


INSERT INTO Inschrijving_masterclass
        VALUES (0, 1, 1)
;
INSERT INTO Inschrijving_masterclass
        VALUES (1, 2, 1)
;
INSERT INTO Inschrijving_masterclass
        VALUES (1, 3, 2)
;


INSERT INTO Masterclass
VALUES (1, 'Masterclass 1', '10.30 uur', '15-2-2012',' Den Haag', 10)
;
INSERT INTO Masterclass
VALUES (2, 'Masterclass 2', '11.30 uur', '16-2-2012',' Den Haag', 11)
;
INSERT INTO Masterclass
VALUES (3, 'Masterclass 3', '12.30 uur', '17-2-2012',' Den Haag', 12)
;


INSERT INTO Speler_masterclass
VALUES (1, 'Speler 1', 'Adres', 'postcode','Den Haag', '0612345678', '[e-mail]', 0)
;
INSERT INTO Speler_masterclass
VALUES (2, 'Speler 2', 'Adres', 'postcode','Den Haag', '0612345678', '[e-mail]', 0)
;
INSERT INTO Speler_masterclass
VALUES (3, 'Speler 3', 'Adres', 'postcode','Den Haag', '0612345678', '[e-mail]', 0)
;

HOWEVER, if I try to do so, I get the following error:

Error code: 1452. Cannot add or update a child row: a foreign key constraint fails.

I have not got a clue what I’m doing wrong. 🙁

Answer 1

When you have foreign key references from one table (source) into another (target), you need to set up the data in that target table first.

At the point where you’re inserting (for example) {5,1,1} into Geeft_masterclass (and those second and third column have foreign key constaints), there is no data in the Speler_masterclass or Masterclass tables, the two tables which are the targets for those constraints.

Quick solution, populate the tables in the same order that you create them. Then the data should exist in the target tables which will allow the constraints to succeed in the source tables:

• Speler_masterclass;
• Masterclass;
• Geeft_masterclass, references Speler_masterclass and Masterclass; and
• Inschrijving_masterclass, references Speler_masterclass and Masterclass.